Thermal analysis and X-ray diffraction of synthesis of scheelite

Article abstract of DOI:10.1023/A:1011577903726

Scheelite (calcium tungstate) is the product of one of the processing methods of wolframite by its roasting with calcium oxide or limestone or its fusion with calcium chloride, followed by acid processing of calcium tungstate with the formation of tungstic acid. Scheelite occurs in contact metamorphic deposits, hydrothermal veins and pegmatites. The present work illustrates a thermal analysis study of synthesis of scheelite by sintering of wolframite with calcite and sintering of tungsten oxide with calcite or calcium oxide using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curve of sintering of wolframite with calcite shows the beginning of the reaction at 560°C with the formation of scheelite. The intensive formation of scheelite is represented by the medium and wide endothermic peak at 740°C. This is followed directly by a large and sharp endothermic peak at 860°C, representing the dissociation of unreacted calcite. The DTA curve of tungsten trioxide shows three thermal effects. The sharp exothermic peak at 320°C represents the oxidation of tungsten oxide content of lower valency. The endothermic peaks at 750 and 1090°C are related to polymorphic changes of tungsten trioxide. The beginning of its sublimation is observed at temperature higher than 800°C. The DTA curves of sintering of tungsten trioxide with calcite or calcium oxide indicate that the intensive formation of scheelite takes place by endothermic reactions at 660 and 545°C respectively. The medium and small endothermic peaks at 520 and 730°C on the DTA curve of tungsten trioxide with calcium oxide represent the dehydration of calcium oxide and the loss of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide from air, respectively. The produced scheelite is colorless in thin sections, has distinct cleavage (101), crystallizes in the tetragonal system in the form of tabular crystals and is optically positive.

Full text of DOI:10.1023/A:1011577903726

Journal of Thermal Analysis and Calorimetry, Vol. 64 (2001) 1283–1296
THERMAL ANALYSIS AND X-RAY DIFFRACTION OF
SYNTHESIS OF SCHEELITE
A. M. Abdel-Rehim
Alexandria University, 69 Sultan Hussein St., Shallalat, Alexandria, Egypt
Abstract
Scheelite (calcium tungstate) is the product of one of the processing methods of wolframite by its
roasting with calcium oxide or limestone or its fusion with calcium chloride, followed by acid pro-
cessing of calcium tungstate with the formation of tungstic acid. Scheelite occurs in contact meta-
morphic deposits, hydrothermal veins and pegmatites.
The present work illustrates a thermal analysis study of synthesis of scheelite by sintering of
wolframite with calcite and sintering of tungsten oxide with calcite or calcium oxide using a
derivatograph. The reaction products were identified microscopically and by using a Siemens Crys-
talloflex diffractometer.
The DTA curve of sintering of wolframite with calcite shows the beginning of the reaction at
560°C with the formation of scheelite. The intensive formation of scheelite is represented by the me-
dium and wide endothermic peak at 740°C. This is followed directly by a large and sharp endother-
mic peak at 860°C, representing the dissociation of unreacted calcite.
The DTA curve of tungsten trioxide shows three thermal effects. The sharp exothermic peak at
320°C represents the oxidation of tungsten oxide content of lower valency. The endothermic peaks
at 750 and 1090°C are related to polymorphic changes of tungsten trioxide. The beginning of its sub-
limation is observed at temperature higher than 800°C.
The DTA curves of sintering of tungsten trioxide with calcite or calcium oxide indicate that the
intensive formation of scheelite takes place by endothermic reactions at 660 and 545°C respectively.
The medium and small endothermic peaks at 520 and 730°C on the DTA curve of tungsten trioxide
with calcium oxide represent the dehydration of calcium oxide and the loss of carbon dioxide due to
some carbonatization of calcium oxide with carbon dioxide from air, respectively.
The produced scheelite is colorless in thin sections, has distinct cleavage (101), crystallizes in
the tetragonal system in the form of tabular crystals and is optically positive.
Keywords: DTA, synthesis of scheelite, XRD
Introduction
Scheelite (calcium tungstate) is the product of one of the processing methods of wolf-
ramite. Direct processing of wolframite by acid leaching is difficult. Therefore, acid
processing of wolframite is intensified by preliminary roasting of wolframite with
calcium oxide or limestone or its fusion with calcium chloride with the formation of
scheelite. This is followed by acid processing of scheelite produced with the forma-
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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ABDEL-REHIM: SYNTHESIS OF SCHEELITE
tion of tungstic acid [1–5]. Scheelite occurs in contact metamorphic deposits, in hy-
drothermal veins, pegmatites and in placer deposits commonly associated with wolf-
ramite [1, 3, 6, 7].
The thermal behaviour of the starting materials of scheelite synthesis, i.e. wolf-
ramite, calcite, tungsten trioxide and calcium oxide, is well known [8–14, 20]. Pure
wolframite does not show any thermal effect at heating up to 1000°C [11]. The DTA
curve of calcite shows its dissociation as is represented by the large and sharp endo-
thermic peak at 830–920°C [2, 8–10]. The DTA curve of lime shows medium and
small endothermic peaks at 525 and 740°C representing its dehydration and the loss
of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide
from air [12, 13]. The DTA curve of tungsten trioxide shows four thermal effects at
300–330, 350–380, 720–750 and 1100°C. The last three effects are reversible and re-
lated to polymorphic changes of tungsten trioxide. The exothermic effect at
300–330°C is irreversible and is due to the oxidation of tungsten oxide content of
lower valency. This is formed by some reduction of tungsten trioxide in the process
of thermal decomposition of ammonium paratungstate [3, 4, 14, 19]. The tungsten tri-
oxide melts at 1470 and boils at 1850°C [3, 4]. Its beginning of sublimation is ob-
served at 800–900°C and its appreciable sublimation at 1200°C [19].
The present work illustrates a differential thermal analysis and X-ray diffraction
study of synthesis of scheelite by sintering of wolframite with calcite and tungsten tri-
oxide with calcite or calcium oxide using a derivatograph. The reaction products
were identified microscopically and by using a Siemens Crystalloflex diffractometer.
Experimental techniques
Mineralogy
The processed wolframite is characterized by black colour and streak in hand-speci-
mens, greasy to submetallic lustre, high specific gravity (7.4) and high hardness.
Crystals are thick tabular to prismatic, with vertical faces often striated parallel to the
c-axis. In polished sections, it is greyish white with faint brownish tint and has long
prismatic to tabular form. Cleavage is often distinct in two directions. Twinning is
common and usually of simple contact type. The crystals show low reflectivity, deep
brownish red internal reflections, weak anisotropism and oblique extinction. Wolf-
ramite crystallizes in monoclinic system and is optically positive.
The X-ray powder diffraction pattern of the studied wolframite sample shows
only the characteristic peaks of wolframite; no mineral impurity was detected. The
X-ray peaks of wolframite are well-defined, narrow and intense, suggesting good
crystallinity, and their data are consistent with those given in the ASTM index.
Chemical composition
The chemical composition of the studied wolframite sample is given in Table 1.
J. Therm. Anal. Cal., 64, 2001

ABDEL-REHIM: SYNTHESIS OF SCHEELITE
1285
Table 1 Chemical composition of wolframite
Chemical component
Content/%
74.81
19.20
4.35
WO3
FeO
MnO
SiO2
SnO2
CuO
CaO
As2O3
S
0.24
0.32
0.11
0.44
0.20
0.25
P2O5
0.05
Procedure
The starting materials of scheelite synthesis usually consisted of wolframite-calcite
and tungsten trioxide-calcite or calcium oxide in particular amounts. Mixes were pro-
cessed by repeated grinding in an automated agate mortar followed by sieving until
all the powder passed through a 200 mesh sieve. Finally, the mixtures were then
ground with a pestle and mortar for 1 h to achieve homogeneity.
Apparatus
Experiments were carried out using ceramic crucibles, heated in an electrical furnace in
air atmosphere. The temperature was regulated automatically with an accuracy ±5°C.
The thermal analysis study of synthesis of scheelite was carried out with a MOM
derivatograph [15]. This apparatus simultaneously records four curves: the change of
temperature of the sample (T), differential thermal analysis (DTA), thermogravi-
metric analysis (TG), quantitatively in mg, and the derivative thermogravimetric
curve (DTG) on a single sample under controlled conditions.
The parameters during the test were as follows: ceramic crucible; inert material
aluminum oxide. Mass of sample 1000 mg; temperature range, ambient up to
1200°C; in air atmosphere; mass used for TG curve 20, 50 and 200 mg, heating rate,
10°C min–1, sensitivity of DTG circuit, 1/3; 1/5 and 1/10; sensitivity of DTG circuit,
1/10. The DTA and temperature thermocouples were Pt–Pt/Rh wires.
Phase identification
The phases of the products of scheelite synthesis were identified microscopically and
by X-ray diffraction analysis, using a Siemens Crystalloflex diffractometer. The
finely ground product of sintering was mixed with sodium chloride as a standard. Its
peaks occurring at 2θ=21.38 and 45.44° were used for corrections. Nickel filtered
copper radiation was used. The exposure time was 1 h. Intensities were collected to a
J. Therm. Anal. Cal., 64, 2001

1286
ABDEL-REHIM: SYNTHESIS OF SCHEELITE
maximum 2θ=70°. The sensitivity of the experiment was 4 104 impl. min–1 and the
statistical error was 1.5%.
Determination of thermodynamic constants
Before the thermal analysis study of scheelite synthesis, an attempt was made to cal-
culate the thermodynamic constants of the reactions. The thermodynamic data given
in Table 2 were used in calculations.
Table 2 Thermodynamic data used
Thermodynamic function/∆F0
FeWO4 (c)
CaWO4 (c)
Calcite
Value/kcal mol–1
–259.800
–376.906
–269.908
–182.630
–144.353
–177.728
–94.260
Reference
[16]
[17, 18]
[17, 18]
[16, 18]
[17, 18]
[17, 18]
[17]
WO3 (c)
CaO (c)
Fe2O3 (c)
CO2 (g)
Synthesis of scheelite via sintering of wolframite with calcite
The reaction may be represented as
2FeWO4+2CaCO3+0.5O2 → 2CaWO4+Fe2O3+2CO2
Scheelite
The standard free energy of the reaction (∆F0) at 25°C is
0
0
0
0
∆Fre0action =2∆F 0
+∆F
+∆F −2∆F
−2∆F
CaWO4
Fe2 O3
CO2
FeWO4
CaCO3
=–2×376.906–177.728−2×94.26+2×259.8+2×269.908
= −1120.06+1059.416=−60.644 kcal mol–1
The equilibrium constant of the reaction (K) may be calculated from the equa-
tion relating to 25°C
∆F0= –RTlnK
−∆F 0
60644
logK=
=
= 44.476
4.57562298 4.57562298
K=2.99 1044
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ABDEL-REHIM: SYNTHESIS OF SCHEELITE
1287
Synthesis of scheelite via sintering of wolframite with calcium oxide
The reaction may be represented as
2FeWO4+2CaO+0.5O2 → 2CaWO4+Fe2O3
The standard free energy of the reaction (∆F0) at 25°C
∆Fre0action =2∆FC0aWO +∆FF0e −2∆FF0eWO −2∆FC0aO
2 O3
4
4
=–2×376.906–177.728−2×259.8+2×144.352
= −931.54+808.304= −123.236 kcal mol–1
123236
logK=
=90.38
4.57562298
K=2.4 1090
Synthesis of scheelite via sintering of tungsten trioxide with calcite
The reaction may be represented as
WO3+CaCO3 → CaWO4+CO2
0
0
0
∆Fre0action =∆F 0
+∆F −2∆F +∆F
CaWO4
CO2
WO3
CaCO3
=–376.906–94.26+182.63+269.908
= −18.628 kcal mol–1
18628
logK=
=13.662
4.57562298
K=4.59 1013
Synthesis of scheelite via sintering of tungsten trioxide with calcium oxide
The reaction may be represented as
WO3+CaO → CaWO4
∆Fre0action =∆FC0aWO −∆FW0O −∆FC0aO
4
3
=–376.906+182.63+144.352
= −49.924 kcal mol–1
logK=36.614
K=4.11 1036
J. Therm. Anal. Cal., 64, 2001

1288
ABDEL-REHIM: SYNTHESIS OF SCHEELITE
The equilibrium constants are large and the reactions of synthesis of scheelite by
sintering of wolframite or tungsten trioxide with calcite or calcium oxide are consid-
ered in practice to be irreversible.
Results and discussion
DTA of synthesis of scheelite by sintering of wolframite with calcite
The thermal analysis data of sintering of wolframite–calcite mix of ratio 1:1.25
(Fig. 1) demonstrate the beginning of the reaction at 560°C with the formation of
scheelite. The intensive formation of scheelite is represented by the medium and wide
endothermic peak at 740°C. This is followed directly by large and sharp endothermic
peak at 860°C, representing the dissociation of unreacted calcite. These processes in-
volve a large and sharp decrease in mass (TG curve), due to the loss of carbon diox-
ide, resulting from the dissociation of calcite.
Fig. 1 DTA curve of synthesis of scheelite by sintering of wolframite–calcite mix of
ratio: 1:1.25. Mass of sample 1000 mg. Heating rate: 10°C min–1
The results obtained are consistent with literature data [1, 3, 4], as the reaction of
wolframite and calcite takes place at 700–775°C with the formation of scheelite.
Microscopic and X-ray diffraction study
The products of the runs at 600 and 740°C during 1 h were identified microscopically
and by using X-ray diffraction.
At 600°C, wolframite and scheelite constitute the main composition of the prod-
uct with hematite and relict grains of unreacted calcite. At 740°C, scheelite consti-
tutes the total composition of the product with hematite and few relict grains of
unreacted wolframite.
J. Therm. Anal. Cal., 64, 2001

ABDEL-REHIM: SYNTHESIS OF SCHEELITE
1289
Fig. 2 X-ray diffraction patterns of the products of scheelite synthesis using wolfram-
ite–calcite mix of ratio 1:1.25. (A, B and C at 600, 740 and 1000°C respec-
tively). Wf – wolframite; C – calcite; Hm – hematite; Sch – scheelite and
F – calcium ferrite
The X-ray diffraction patterns of the products at 600, 740 and 1000°C are shown
in Figs 2 A, B and C respectively. At 600°C, the product of sintering consists mainly
of wolframite and scheelite with hematite and undissociated calcite. This indicates
the low rate of the reaction between wolframite and calcite. At 740°C, scheelite is ob-
served to be the main constituent of the product with hematite. The X-ray peaks of
wolframite disappear completely, indicating that the relict grains of unreacted wolf-
ramite is out of the detection limit of X-ray diffraction. At 1000°C, scheelite consti-
tutes the total composition of the product of this run with calcium ferrite of composi-
tion Ca2Fe2O5. The X-ray peaks of scheelite are well defined, sharp and intense, sug-
gesting good crystallinity.
Differential thermal analysis of tungsten trioxide
Before the thermal analysis study of scheelite synthesis by sintering of tungsten triox-
ide with calcite or calcium oxide, the thermal behaviour of tungsten trioxide was
studied. The DTA curve of tungsten trioxide (Fig. 3) shows small endothermic peak
at 90°C, representing the loss of moisture content. The medium and sharp exothermic
peak at 320°C represents the oxidation of tungsten oxide content of lower valency.
J. Therm. Anal. Cal., 64, 2001

1290
ABDEL-REHIM: SYNTHESIS OF SCHEELITE
The wide endothermic peak at 750°C represents phase transformation from alpha
form to beta-one. The small endothermic peak 1090°C represents the polymorphic
change of tungsten trioxide from beta to gamma-form. There is no change of the mass
of the sample (TG curve). The mass is slightly decreased at temperature higher than
800°C representing the beginning of sublimation of tungsten trioxide.
Fig. 3 DTA curve of tungsten trioxide. Mass of sample 120 mg; heating rate:
10°C min–1 and DTA: 1/3
The thermal data obtained of tungsten trioxide agreed well with the literature
data [3, 4, 14, 19, 20], as the oxidation of tungsten oxide content of lower valency as-
sociated with tungsten trioxide takes place at 330°C. Tungsten trioxide has three
crystalline forms. Alpha-form is stable up to 720–750°C, beta-form stable in the tem-
perature range 720–1100°C and gamma-form stable at temperature higher than
1100°C. The thermal effects of these polymorphic changes are reversible [4, 14, 19].
The beginning of sublimation of tungsten trioxide is observed at temperature range
800–900°C [19] or precisely at 850°C [3] and the rate of sublimation increases at
1200°C, reaching 35% during 10 min [3, 19].
DTA of synthesis of scheelite by sintering of tungsten trioxide with calcite
The thermal analysis data of sintering of tungsten trioxide–calcite mix of ratio 1:1.25
(Fig. 4) show medium and sharp exothermic peak at 330°C, representing the oxida-
tion of tungsten oxide content of lower valency. The beginning of the reaction be-
tween tungsten trioxide and calcite is observed at 520°C. The intensive formation of
scheelite is represented by the wide endothermic peak at 660°C. The phase transfor-
mation of tungsten trioxide from alpha to beta-form at 750°C is masked by the de-
J. Therm. Anal. Cal., 64, 2001

ABDEL-REHIM: SYNTHESIS OF SCHEELITE
1291
Fig. 4 DTA curve of synthesis of scheelite by sintering of tungsten trioxide–calcite mix
of ratio: 1:1.25. Mass of sample 1070 mg; heating rate: 10°C min–1
composition of the unreacted calcite. This is represented by the large and sharp endo-
thermic peak at 860°C.
The TG curve shows a marked decrease in mass in three steps. The first is the
loss of carbon dioxide from the reaction of sintering of tungsten trioxide with calcite
with the formation of scheelite. The second is due to the liberation of carbon dioxide,
resulting from calcite dissociation. The third decrease of mass is observed at 900°C
due to slight sublimation of tungsten trioxide. The sublimation of tungsten trioxide
does not exceed 0.1–0.2% at 900°C in air atmosphere [19].
Microscopic and X-ray diffraction study
The products of the runs at 520, 660 and 860°C during 1 h were identified microscop-
ically and by using X-ray diffraction.
At 520°C, the product of sintering is composed of scheelite with large amount of
unreacted tungsten trioxide and calcite. This indicates the low rate of the reaction at
such temperature. At 660°C, scheelite is the main constituent of thin section, together
with some unreacted tungsten trioxide and calcite grains. This indicates high rate of
the reaction and intensive formation of scheelite, but the incompleteness of the reac-
tion is due to the short time. Scheelite appears in thin sections as colorless tabular
crystals. At 860°C, scheelite constitutes the total composition of the product with a
very few residual grains of tungsten and calcium oxides.
The X-ray diffraction patterns of these products are shown in Figs 5 A, B and C
at 520, 660 and 860°C, respectively. At 520°C, the product shows the beginning of
the reaction and the appearance of scheelite with considerable amount of unreacted
J. Therm. Anal. Cal., 64, 2001

1292
ABDEL-REHIM: SYNTHESIS OF SCHEELITE
Fig. 5 X-ray diffraction patterns of the products of scheelite synthesis using tungsten
trioxide–calcite mix of ratio 1:1.25. (A, B and C at 520, 660 and 860°C respec-
tively). W – tungsten trioxide; C – calcite and Sch – scheelite
tungsten trioxide and calcite. At 660°C, scheelite constitutes the major component of
the sintering product with small amount of tungsten trioxide and calcite.
The X-ray diffraction pattern of the end product at 860°C shows only the schee-
lite peaks, indicating that scheelite constitutes the total composition of the product.
The tungsten trioxide and calcite peaks disappeared completely, indicating complete-
ness of the reaction and all tungsten trioxide and calcite are consumed in the forma-
tion of scheelite. The X-ray peaks of scheelite are sharp, well-defined and intense,
suggesting good crystallinity. The X-ray diffraction study of the products of sintering
is in good agreement with microscopic study of their thin sections.
DTA of synthesis of scheelite by sintering of tungsten trioxide with calcium oxide
The thermal analysis data of sintering of tungsten trioxide–calcium oxide mix of ratio
1.1.25 (Fig. 6) shows a medium and sharp exothermic peak at 330°C, representing the
oxidation of some tungsten oxide content of lower valency. The small endothermic
peak at 520°C represents the dehydration of some calcium hydroxide content in cal-
cium oxide due to its partial hydration. The wide and large endothermic peak at
545°C represents the intensive formation of scheelite. The endothermic peak at
730°C represents the loss of carbon dioxide due to some carbonatization of calcium
J. Therm. Anal. Cal., 64, 2001

ABDEL-REHIM: SYNTHESIS OF SCHEELITE
1293
Fig. 6 DTA curve of synthesis of scheelite by sintering of tungsten oxide–calcium ox-
ide mix of ratio: 1:1.25. Mass of sample 906 mg; heating rate: 10°C min–1 and
DTA: 1/5
oxide with carbon dioxide from air. The phase transition of tungsten trioxide from al-
pha- to beta-form at 750°C is masked by the effect of loss of carbon dioxide. The
small endothermic peak at 1090°C represents the phase transition of tungsten trioxide
from beta to gamma-form.
The TG curve shows a marked decrease of mass in four steps. The first step is
the loss of moisture content at 100°C. The second loss at 520°C is due to the dehydra-
tion of hydrated calcium oxide content. The third decrease of mass is observed at
730°C representing the loss of carbon dioxide due to some carbonatization of calcium
oxide. The fourth step is the loss of mass at temperature higher than 800°C, which is
due to the beginning of sublimation of tungsten trioxide.
The results obtained agreed very well with the data in the literature [12–14], as
the dehydration of calcium hydroxide has been reported to take place at 450–525°C
and the loss of carbon dioxide (bound by calcium oxide from air) at 660–730°C. The
reaction of tungsten trioxide and calcium oxide occurs at 500–600°C, with the forma-
tion of calcium tungstate or scheelite [3, 4].
The microscopic study of the products of the runs at 500 and 550°C for 2 h show
that scheelite is the main phase of the product at 500°C with some tungsten and cal-
cium oxide grains. At 550°C, scheelite constitutes the total composition of the end
product. The resulting scheelite is white with yellowish tint, colorless in thin sections
and crystallizes in the tetragonal system, in the form of tabular crystals, with distinct
(101) cleavage. The X-ray diffraction pattern of the product at 550°C (similar to
Fig. 5 C) shows only the presence of scheelite.
This study of the conditions of synthesis and formation of scheelite and its ther-
mal stability illustrates the thermal characteristics of processing of wolframite by its
sintering with calcite and has provided good evidence concerning its genesis and oc-
currence in contact metasomatic deposits.
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1294
ABDEL-REHIM: SYNTHESIS OF SCHEELITE
Table 3 X-ray powder diffraction data of scheelite
d/Å
I/I0/%
h k l
ASTM
4.76
Observed
4.764
3.110
3.060
2.863
2.623
2.296
2.253
2.089
1.994
1.928
1.852
1.727
1.688
1.633
1.592
1.554
1.441
1.421
1.387
1.357
1.336
1.311
1.264
1.249
1.229
1.208
1.205
1.190
1.172
ASTM
53
100
31
14
23
19
3
Observed
65
100
35
15
21
20
3
101
112
103
004
200
211
114
105
213
204
220
310
116
215
312
224
321
008
305
323
217
400
411
316
109
332
413
404, 307
420
3.10
3.072
2.844
2.622
2.296
2.256
2.0864
1.9951
1.9278
1.8538
1.7278
1.6882
1.6332
1.5921
1.5532
1.4427
1.4219
1.3859
1.3577
1.3358
1.3106
1.2638
1.2488
1.2284
1.2074
1.2054
1.1901
1.1728
5
7
13
28
12
5
12
35
13
5
10
10
30
14
6
12
12
27
12
6
2
2
3
3
4
5
3
3
3
3
2
2
13
2
12
2
5
5
5
5
4
3
1
1
General characteristics of the synthesized scheelite
The produced scheelite is white with a yellowish tint in hand-specimen and has an ad-
amantine lustre. In thin sections, scheelite is colorless, has distinct (101) cleavage,
crystallizes in tetragonal system in the form of tabular crystals and is optically posi-
J. Therm. Anal. Cal., 64, 2001

ABDEL-REHIM: SYNTHESIS OF SCHEELITE
1295
tive. In polished sections, scheelite is dark-grey with very low reflectivity, yellowish
white internal reflections and distinct anisotropism.
The synthesized scheelite has the following chemical composition: 80.58%
WO3 and 19.40% CaO.
The X-ray diffraction pattern of the produced scheelite (Fig. 5 C) shows the
characteristic and well-defined peaks of scheelite, which are sharp and intense, sug-
gesting good crystallinity. The X-ray diffraction data of the synthetic scheelite are
consistent with the corresponding ASTM values of the natural mineral (Table 3).
Table 4 Unit cell dimensions and axial angles of the synthesized scheelite
Mineral
a/Å
b/Å
(Tetragonal)
5.2376
c/Å
α/degree β/degree
γ/degree
V/Å3
Scheelite
Synthetic
5.2376
±0.0113
5.246
11.3458
±0.0103
11.349
90.00
0.00
90.00
0.00
90.00
0.00
311.243
±0.214
±0.0113
5.246
Standard
90.00
90.00
90.00
312.330
The unit cell dimensions and constants of the synthetic scheelite are given in Ta-
ble 4. It is observed that the calculated cell dimensions, constants and optic axial an-
gles of the synthesized scheelite are consistent with the corresponding data of the nat-
ural mineral.
Conclusions
The thermal analysis and X-ray diffraction study of synthesis of scheelite by sintering
of wolframite with calcite and sintering of tungsten trioxide with calcite or calcium
oxide has revealed the following conclusions:
1. The DTA curves of sintering of wolframite with calcite show the beginning
of the reaction at 560°C with the formation of scheelite. The intensive formation of
scheelite is represented by the medium and wide endothermic peak at 740°C. This is
followed directly by large and sharp endothermic peak at 860°C, representing the dis-
sociation of the unreacted calcite.
The standard free energy (∆F0) and equilibrium constant of the reaction (K) at
298 K are –60.644 kcal mol–1 and 2.99 1044 respectively. These values reflect the irre-
versibility of the reaction.
2. The DTA curve of tungsten trioxide shows three thermal effects. The sharp
exothermic peak at 320°C represents the oxidation of tungsten oxide content of lower
valency. The endothermic peaks at 750 and 1090°C are related to polymorphic
changes of tungsten trioxide. The beginning of its sublimation is observed at temper-
ature higher than 800°C.
3. The DTA curves of sintering of tungsten trioxide with calcite indicate that
the intensive formation of scheelite takes place at 660°C by endothermic reaction.
The phase transition of tungsten trioxide at 750°C is masked by the dissociation of
calcite at 860°C.
J. Therm. Anal. Cal., 64, 2001

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ABDEL-REHIM: SYNTHESIS OF SCHEELITE
4. The DTA curves of sintering of tungsten trioxide with calcium oxide shows a
wide endothermic peak at 545°C, representing the formation of scheelite. The me-
dium and small endothermic peaks at 520 and 730°C represent the dehydration of cal-
cium oxide and the loss of carbon dioxide due to some carbonatization of calcium ox-
ide with carbon dioxide from air.
The synthesized scheelite is colorless in thin sections, has distinct (101) cleav-
age, crystallizes in the tetragonal system in the form of tabular crystals and is opti-
cally positive.
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